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Principle of NMR Spectroscopy

NMR-Spectroscopy Principle 

NMR spectroscopy involves the absorption of electromagnetic radiation in the radiofrequency region and the principle of NMR spectroscopy revolves around the non- zero value of nuclear spin i.e. I

 

S. No.

Number of Protons

Number of Neutrons

Nuclear Spin (I)

examples

1

even

even

0

6C12, 8O16

2*

            odd

even

Half integer

(1/2, 3/2, 5/2)

1H1, 9F19, 15P31

3**

            even

odd

Half integer

(1/2, 3/2, 5/2)

6C13

4***

             odd

odd

Full integer

7N14

*This is possible when both atomic number and mass number are odd.

 ** This is possible when both atomic number is even and mass number is odd.

 *** This is possible when both atomic number is odd and mass number is even.

 

In the absence of an external magnetic field, these spins are randomly oriented. However, on application of external magnetic field, their spins align in different possible orientations by aligning themselves either in the direction of the external magnetic field or in the direction opposite to the external magnetic field.

            Number of possible orientations = 2I+1

For I= ½ the possible orientations = 2x ½ +1 = 2


These spins are now separated by a fixed energy difference between them which is denoted by ΔE and this difference varies directly with the external applied magnetic field i.e. B0.

                                                                     ΔE  B0      and  ΔE  γ and

where, ΔE = energy difference between different spin states
 B0  = external applied magnetic field
h =  Planck's constant
ν = Precessional Frequency

γ = Gyromagnetic Ratio = magnetic moment /angular momentum

   ΔE = hν this makes, ν = γB0/2π

the interaction of spinning nucleus (smaller magnet) with the applied magnetic field generates a precessional motion in the spinning nucleus having a precessional frequency, ν. When this precessional frequency matches with frequency of sources’s radiowaves, the transition occurs from lower energy sate to higher. That’s why the process is known as Nuclear Magnetic Resonance. This absorption is recorded in the form of an NMR spectrum which provides significant information.




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